The sum of the three angles in a triangle is 180 degrees. The sum of two angles making a line is also 180 degrees: that leads to the well known theorem (it's probably in your text) that the measure of an external angle (here the one marked "125") is equal to the sum of the measures of the two opposite angles. One of those two opposite angles is 50 degrees. From that you can find the measure of the angle between "80" and "x" and then find x.